EM spectrum: radio waves, microwaves, infrared and visible light
The long-wavelength end of the spectrum holds radio waves, microwaves and infrared, then the narrow visible band, 700 nm (red) to 400 nm (violet).
The electromagnetic spectrum
All EM waves are the same kind of wave. They differ only in frequency and wavelength. Arranged in order of frequency, they form the electromagnetic spectrum. In vacuum all of them travel at the same speed c, so c = νλ: a higher frequency means a shorter wavelength. The regions have no sharp boundaries, and neighbouring regions overlap. The names come mainly from how the waves are made or detected. This topic covers the long-wavelength end: radio waves, microwaves, infrared and visible light.
Radio waves to visible light
| Wave | Wavelength | Produced by | Detected by | Main uses |
|---|---|---|---|---|
| Radio waves | More than 0.1 m | Accelerated motion of charges in conducting wires (aerials) | Receiver aerials | AM (530 kHz to 1710 kHz) and FM (88 MHz to 108 MHz) radio, TV, mobile phones (UHF band) |
| Microwaves | 0.1 m to 1 mm | Special vacuum tubes (klystrons, magnetrons) and Gunn diodes | Point-contact diodes | Radar for aircraft navigation, speed guns, microwave ovens |
| Infrared | 1 mm to 700 nm | Hot bodies; vibrating atoms and molecules | Thermopiles, bolometers, infrared film | TV remotes, physiotherapy lamps, satellite study of crops, keeping Earth warm (greenhouse effect) |
| Visible light | 700 nm to 400 nm | Electrons in atoms moving to lower energy levels | The eye, photocells, photographic film | Seeing, photography |
Elementary facts about their uses
- Radio waves carry radio and TV programmes. Mobile phones use radio waves in the UHF (ultra high frequency) band to carry voice.
- Microwaves used in radar reflect off aircraft and help guide them. Police speed guns use the same idea.
- In a microwave oven, the frequency matches a natural (resonant) frequency of water molecules. Water in the food absorbs the energy, its molecules move faster, and the food heats up.
- Infrared is readily absorbed by the water molecules in most materials, which warms them. So it is called heat radiation.
- Earth's surface gives out infrared. Greenhouse gases trap part of it and keep Earth warm: the greenhouse effect.
- Visible light is the band our eyes detect. Some animals sense other bands: snakes can detect infrared, and many insects see into the ultraviolet.
Worked example: wavelength and region
Find the wavelength and name the region for (a) an FM station at 100 MHz, (b) a radar at 10 GHz and (c) light of frequency 5 × 10¹⁴ Hz. (c = 3 × 10⁸ m s⁻¹)
- Formula: λ = c/ν.
- (a) λ = 3 × 10⁸ ÷ (100 × 10⁶) = 3 × 10⁸ ÷ 10⁸ = 3 m: radio waves.
- (b) λ = 3 × 10⁸ ÷ (10 × 10⁹) = 3 × 10⁸ ÷ 10¹⁰ = 3 × 10⁻² m = 3 cm: microwaves.
- (c) λ = 3 × 10⁸ ÷ (5 × 10¹⁴) = 0.6 × 10⁻⁶ m = 600 nm: visible light (orange).
(a) 3 m, radio; (b) 3 cm, microwave; (c) 600 nm, visible
Your TV remote sends infrared pulses, and FM radio and mobile phones use radio waves. Indian remote-sensing satellites use infrared sensors to watch how crops are growing across the country.
1 marks: “Which EM waves are used in radar systems?” Microwaves.
2 marks: “Name the EM waves (a) produced by a klystron or magnetron and (b) used in TV remote controls. Give one more use of each.” (a) Microwaves: microwave ovens or speed guns. (b) Infrared: physiotherapy lamps or satellite study of crops.
3 marks: “Write the wavelength range, one way of producing and one use of radio waves, microwaves and infrared waves.” Radio: more than 0.1 m, charges accelerated in aerials, radio and TV. Microwave: 0.1 m to 1 mm, klystron or magnetron, radar. Infrared: 1 mm to 700 nm, hot bodies, TV remotes.
Increasing frequency: Raju Makes Ice Very Useful, eXtra Good (Radio, Microwave, Infrared, Visible, UV, X-rays, Gamma).